Ultrafast optical manipulation of atomic arrangements in chalcogenide alloy memory materials

A class of chalcogenide alloy materials that shows significant changes in optical properties upon an amorphous-to-crystalline phase transition has lead to development of large data capacities in modern optical data storage. Among chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used because of its reliability. We use a pair of femtosecond light pulses to demonstrate the ultrafast optical manipulation of atomic arrangements from tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST superlattices. Depending on the parameters of the second pump-pulse, ultrafast nonthermal phase-change occurred within only few-cycles (~ 1 ps) of the coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast switch in chalcogenide alloy memory could lead to a major paradigm shift in memory devices beyond the current generation of silicon-based flash-memory.


Introduction
Chalcogenide alloys are semiconducting materials with periodic and random bonding networks, whose reversible phase transition enabled electrically driven non-volatile memory devices [1,2]. Multi-component chalcogenides, such as Ge-Sb-Te and Ag-In-Sb-Te, are potentially used in optical data storage media in the forms of rewritable CDs, DVDs, and Bluray discs [2][3][4]. Especially, Ge 2 Sb 2 Te 5 (GST) has proven to be one of the highest-performance alloys among commercially available phase-change materials, whose reliability allows more than 10 5 write-erase cycles [3][4][5][6]. The process of a rapid phase change involved in the writing and erasing of data is induced by the irradiation of focused nanosecond laser pulses, leading to transient temperature ramping [3][4][5][6]. Thus, the dynamics of the rapid phase change in the optical recording media is generally governed by a thermal (incoherent) process, limiting the speed of write-erase cycles to the MHz-GHz range [4,5].
A question arising from the dynamics of the phase change in chalcogenide alloys is how fast the phase transformation between the amorphous and crystalline phases occurs. Motivated by understanding the mechanism of the rapid phase change process, extensive investigations on GST have been carried out using optical absorption [7], Raman scattering and x-ray absorption fine structure (XAFS) measurements [8], and the ab initio molecular dynamics and density functional calculations [6,9,10]. However, the kinetics of the rapid phase-change in GST alloys has still been actively debated, limiting the acceleration of its switching speed.
Coherent phonon spectroscopy (CPS) is a powerful tool to study the ultrafast dynamics of structural phase transitions occurring on ultrafast time scales, as has been applied to a variety of materials, such as semimetal and semiconductors [11,12], and Mott insulators [13,14]. Lattice dynamics in GST alloys and atomically controlled superlattice (SL) films was examined by using a femtosecond pump-probe technique and found that the appearance of the coherent vibrational modes was significantly modified upon the phase change [15,16].
However, yet optical manipulation of atomic arrangements in GST materials has not been reported.
Here we demonstrate in GST superlattice (SL) [17][18][19] that the phase change from amorphous into crystalline states can be manipulated by controlling atomic motions through selectively exciting a vibrational mode that involves Ge atoms, i.e., the local GeTe 4 vibrational mode in the amorphous phase. Importantly, the phase change occurs only when the time separation between the two pump-pulses is simultaneously resonant to the local vibration. In addition, the transient frequency of the local vibrational mode initially softened on a subpicosecond time scale, followed by the quenching at different final states, depending on the fluence of the second pump-pulse, to which we interpret ultrafast crystallization.

Experimental details
Recently, Chong et al. proposed the superlattice-like phase-change random access memory (PCRAM) on the basis of considering the GST system as pseudo-binary GeTe and Sb 2 Te 3 alloys, such that Ge 2 Sb 2 Te 5 ! " (GeTe) 2 (Sb 2 Te 3 ) [17]. Both the faster switching time (< 5ns) and lower programming current have been found in the superlattice-like PCRAM. More recently, Tominaga et al. reported on the fabrication of a GST superlattice (SL) based upon a Ge flip-flop transition mechanism [8,18,19]. Using the GST-SL films artificially designed from GeTe sub-layers and Sb 2 Te 3 sub-layers, they experimentally confirmed a very low (only ≈ 10 % compared to GST alloy films) power operation of the phase switching (amorphous ! " cubic) in the GST-SL, implying that GST-SL have an ideal structure to realize the flip-flop transition of Ge atoms. A sample used in this paper was thin films of GST-SL, which consisted of twentyrepetitive sheet blocks from alternatively deposited 0.5-nm-thick GeTe and Sb 2 Te 3 sub-layers on a Si-(100) wafer at room temperature using a helicon-wave RF magnetron-sputtering machine [ Fig. 1(a)]. The phase of the as-grown GST-SL films was confirmed as amorphous by both transmission electron microscopy (TEM) and electron-diffraction measurements. By annealing the as-grown GST-SL films at 503 K for ten minutes, the GST-SL film was transformed from the amorphous into crystalline states [16,18], where the layered structure of GST-SL films has been confirmed by high-resolution TEM measurements [19].
We utilized a 20-fs near-infrared optical pulse (1.46 eV) to excite coherent lattice vibrations in GST-SL films after injection of photo-carriers across the indirect band gap of 0.6 -0.7 eV [7]. The photo generated carrier density was estimated to be n exc ≈ ! 4.0 " 10 19 cm -3 , induced by the single pump-pulse with 64 µJ/cm 2 . A couple of the pump-pulses (pump-pulse pair) were generated through a Michelson-type interferometer, where a piezo-stage was equipped under the mirrors to adjust the time interval (Δt) of the temporally separated pumppulses to a precision of < 0.1 fs [ Fig. 1(b)]. The photoinduced reflectivity change (ΔR/R) was recorded as a function of the time delays between the pump and probe pulses at room temperature (295 K). We utilized a discrete wavelet transform (DWT) method [20] with a Gaussian time window to explore the transient frequency of the coherent vibrations. The coherent vibration observed in the amorphous GST-SL film is compared with that observed in the crystalline phase under the single pump-pulse excitation with 64 µJ/cm 2 as shown in Fig. 2(a), which is the photoinduced reflectivity change (ΔR/R). As seen in the corresponding Fourier transformed (FT) spectra presented in Fig. 2(b), the coherent phonon spectra obtained from the GST-SL film in the amorphous phase exhibits two peaks, one at ≈ 3.84 THz that originated from the local A 1 mode of the tetrahedral GeTe 4 species and another mode at ≈ 5.0 THz, which likely corresponds to the A 1 optical mode due to pyramidal SbTe 3 in the Sb 2 Te 3 sub-layers [15,16,21]. Note that the stronger peak at 3.84 THz dominates the coherent phonon spectrum in the amorphous phase, unlike in Raman measurements [21] and density functional study [22], in which a broad Raman peak appeared at ≈ 4.5 THz (= 150 cm -1 ). The difference in the phonon spectra would be attributed to the sample structure and the excitation mechanism of the coherent A 1 mode of the tetrahedral GeTe 4 in GST-SL; we found a pump-polarization dependence of the amplitude of the coherent A 1 mode (3.84 THz) (not shown). This may suggest a possibility of anisotropic excitation of carriers from bonding into anti-bonding states, which exerts a force on the lattice driving the coherent A 1 mode. In the crystalline phase of the GST-SL film, on the other hands, a single sharp peak appears at ≈ 3.68 THz. For Ge 2 Sb 2 Te 5 superlattice, the frequency of the local A 1 mode in the crystalline phase (3.68 THz) is significantly lower than that in the amorphous phase, owing to the local structural change from tetrahedral GeTe 4 into octahedral GeTe 6 species [see the inset of Fig.  2(b)]. Thus, the difference between the amorphous and crystalline phases in the GST-SL film has been clearly revealed as the peak positions of the two characteristics FT spectra.

Fluence dependence of the coherent A 1 modes with single pump-pulse irradiation
Before performing the present experiments using the pump-pulse pair, we first examined the pump fluence dependence of the coherent local A 1 mode by the irradiation of the single pumppulse to check if the phase change from the amorphous into crystalline states is possible. The coherent phonon signal are recorded as a function of the time delay as shown in Fig. 3(a), and the corresponding FT spectra are obtained as shown in Fig. 3(b). At the lowest fluence (47 µJ/cm 2 ) the coherent phonon spectrum exhibits similar structure as in Fig. 2(b) (amorphous), where two peaks appear, one is at 3.84 THz and the other is at ≈ 5.0 THz. As the pump fluence increases, the peak frequency of the A 1 mode of GeTe 4 very slightly red-shifts from 3.84 THz to 3.80 THz, while that of the SbTe 3 pyramids red-shifts from ≈ 5.0 to 4.73 THz. Moreover, it is found that the ratio of the peak intensity I SbTe3 /I GeTe4 significantly increases with increasing the pump fluence, implying the energy deposited by the laser irradiation goes mainly into the A 1 mode of SbTe 3 pyramids. The small peak shift (only ≈ 0.04 THz) observed for the A 1 mode of GeTe 4 units cannot be accounted for the phase change since the peak frequency of the A 1 mode in the crystalline phase should be 3.68 THz as shown in Fig. 2(b). From the results of the single pump-pulse irradiation, we have concluded that the phase change from the amorphous into crystalline states is difficult by this single pump-pulse excitation. Thus, the experiments using the pump-pulse pair has been motivated by concentrating the energy deposited by the laser irradiation on the coherent A 1 mode of GeTe 4 in order to induce the phase change.

The effect of the pump-pulse pair excitation on the relaxation time of coherent phonons
To see the effect of the pump-pulse pair excitation, we first compare the relaxation time (τ) of the A 1 mode (due to the local GeTe 4 structure) observed by the single pump-pulse excitation to that by the pump-pulse pair excitation with the separation time of Δt = 276 fs. Here, Δt = 276 fs is close to the time period of the local A 1 mode of the octahedral GeTe 6 , as is described in the next section.   Fig. 4(b)], in which the fit of the data with damped harmonic oscillations [23] gives τ = 890 fs, while that observed by the pump-pulse pair with Δt = 276 fs is significantly longer (τ = 1080 fs) than that in the case of the single pump-pulse [Figs. 4(c) and (d)]. Since the relaxation process of the A 1 mode (GeTe 4 ) in the amorphous phase has found to not depend on the lattice temperature, but depended mainly on the defect density [16], the prolongation of the relaxation time upon the pump-pulse pair excitation cannot be explained by the effect of lattice heating. Hence, this result indeed suggests a possibility of phase change from amorphous into crystalline state induced by coherent excitation because relaxation of phonon modes in crystalline phase becomes longer than that in the amorphous phase, due to the reduction of phonon-defect scattering in the crystalline phase [16,24].

Coherent manipulation of coherent phonons by tuning the pump fluence and time interval
To check if the coherent excitation of the local lattice vibration induced subsequent phase change, following two procedures were examined using the amorphous GST-SL film. The first parameter varied was the fluence of the second pump-pulse (F 2 ) to determine whether critical laser fluence existed above which the phase change was induced [Figs. 5(a) to 5(c)]. In this experiment, the time interval (Δt) between the first and the second pump-pulse was fixed at 276 fs (corresponds to 3.62 THz), which is intentionally close to the time period of the local A 1 mode of the octahedral GeTe 6 (in crystalline phase), allowing the second pump-pulse to constructively enhance the vibrational amplitude toward the crystalline phase [ Fig. 5(a)] [23,25]. Most remarkably, as a result, the peak frequency significantly shifts from 3.83 THz (at F 2 = 0; single pump) down to 3.68 THz (at F 2 = 64 µJ/cm 2 ) as the value of F 2 is increased [Figs. 5(b) and 5(c)]. Here the FT spectra in Fig. 5(b) are obtained from the time-domain data in Fig.  5(a) just after the second pump-pulse to exclude any artifact originated from the periods of the transient electronic response induced by the pump-pulse pair.
In the context of Figs. 5(b) and 5(c), a vibrational frequency shift due to anharmonic effects would play a minor role, since the pump fluence is less than ≈ 10 -2 of that needed to induce coherent anharmonic vibrations [26]. Furthermore, the origin of the frequency shift is not lattice heating, because the lattice heating should generally occur on several picosecond time scales [27]. Consequently, the frequency shift, from Ω A to Ω C , observed by enhancing the vibrational amplitude is attributed to the ultrafast crystallization. Note that it is natural that the A 1 mode at ≈ 5.0 THz (SbTe 3 mode) is weakly preserved after the crystallization, because the GST-SL film holds the pyramidal SbTe 3 bonds in both phases if the mixing of sub-layers does not occur [18,19,21].
As the second procedure, the dependence of the phonon frequency on the time interval Δt of the pump-pulse pair is an intriguing way to explore whether the ultrafast phase change in GST-SL film is stimulated by a coherent phenomenon. Here, the fluences of the first and the second pump-pulses were fixed at 76 and 64 µJ/cm 2 , respectively, and Δt was varied from 141 to 276 fs. For the shortest time interval [Δt =141 fs in Fig. 5(d)], the amplitude of the coherent vibrations is suppressed by the second pump-pulse because of the destructive interference between the firstly and the secondly generated coherent vibrations by each pump-pulse [23,25]. On the contrary, as Δt increases up to 276 fs, the signal is enhanced after the irradiation of the second pump-pulse due to the constructive interference [23,25]. The corresponding FT spectra [ Fig. 5(e)] revealed a red-shift of the peak frequency, depending on the value of Δt; the frequency of the A 1 mode decreases toward that of the crystalline phase (≈ 3.68 THz) as Δt approaches 276 fs [ Fig. 5(f)]. These data strongly indicate that the coherent (constructive) excitation of the local A 1 mode is crucial to induce the ultrafast phase change. The arrow indicates the shift of the local A1 mode (GeTe4), which is more visible in the variation of the frequency as the function of F2 in (c). The dotted lines in (c) correspond to the frequencies of the local A1 mode (GeTe6) in the crystalline phase (= 3.68 THz) and that (GeTe4) in the amorphous phase (= 3.83 THz), respectively. The solid curve is the guide for the eyes. (d) Coherent vibrations excited by the pump-pulse pair as Δt is varied. (e) FT spectra from the time-domain data in (d). The FT spectrum at Δt = 141 fs is omitted since the intensity is too low to display. The arrow indicates the peak-shift of the local A1 mode (GeTe4), being more visible in (f), which represents the frequency of the local A1 mode as the function of the time interval Δt. The solid curve is the guide for the eyes.